Trackballs are sensitive to solid particles or liquids likely to pollute the surface of the movable part. Such pollution requires the equipment comprising the trackball to be removed which causes the mean time between two failures to fall. Several solutions may be envisaged to improve the reliability of an item of equipment comprising a trackball. An attempt has been made for example to improve the seal of the trackball itself. This seal is situated between the movable part and the fixed part of the trackball. Improving this seal requires a greater tightening of the seals. Tightening restricts the movement of the sphere relative to the body and causes jerky motions that are bad for its operation. Another solution consists in replacing mechanical elements rotating in contact with the sphere and used as sensors with optical sensors directly tracking the movement of the sphere. These optical sensors are less sensitive to pollutants but are of a more complex technology and tend to markedly increase the cost of the trackball.